The present invention relates to gas turbine cycles employing a cooled cooling air (CCA) system for cooling hot compressor air extracted for cooling turbine or compressor parts.
In modern high efficiency gas turbine cycles, hot compressor air is extracted for cooling compressor and turbine parts. Reliability concerns, control difficulty and the high cost of current CCA systems have made it apparent that there is a need for an improved CCA system design.
FIG. 1 schematically illustrates a combined cycle system having a state of the art cooled cooling air system (hereafter CCA system) for cooling hot compressor air extracted for cooling turbine and compressor parts. In the illustrated power plant, a gas turbine system 10 is provided comprising a compressor 14, a combustion system 18 and a gas turbine expander 24; and a steam turbine system schematically shown at 34. More specifically, ambient air 12 enters the axial flow compressor 14 and the thus produced compressed air 16 enters the combustion system 18 where fuel 20 is injected and combustion occurs. The combustion mixture 22 leaves the combustion system 18 and enters the turbine 24. In the turbine section, energy of the hot gases is converted into work. This conversion takes place in two steps, the hot gases are expanded and a portion of the thermal energy is converted into kinetic energy in the nozzle section of the turbine. Then, in the bucket section of the turbine, a portion of the kinetic energy is transferred to the rotating buckets and converted to work, e.g. rotation of shaft 26. A portion of the work developed by the turbine is thus used to drive the compressor 14, whereas the remainder is available for, e.g., an electrical generator or mechanical load 28. Hot exhaust gas 30 leaves the turbine and flows to a heat recovery unit. The heat recovery unit may take the form of any one of a variety of known heat exchange systems including, for example, an otherwise conventional multi-pressure heat recovery steam generator (HRSG) 32.
In the illustrated configuration, the gas turbine system 10 and the steam turbine system 34 each drive a respective generator (or other load) 28, 36. The steam turbine system 34 is associated in a conventional manner with the multi-pressure HRSG 32. Thus, steam 38 flows to/from the steam turbine system 34, the steam turbine system 34 exhausts to a condenser 40, and condensate is fed from condensor 40 to HRSG 32 via conduit 42 with the aid of a condensate pump 44. The condensate passes through the various components of the HRSG 32. Only the low pressure evaporator 46, intermediate pressure evaporator 48 and high pressure evaporator 50 are illustrated in this example, it being well understood that various economizers, superheaters and associated conduits and valves are conventionally provided in an HRSG and are simply omitted as not directly relevant to the discussion herein.
As noted above, heat is provided to the HRSG 32 by the gas turbine exhaust gases 30 that are introduced into the HRSG 32 and exit the HRSG at 52 for passage to a stack (not shown). The further discussion of this conventional system will be generally limited to those components provided as a part of the associated CCA system 54.
In the illustrated CCA system 54, hot compressor air is extracted, as schematically shown by conduit 56, at temperatures of for example 850 to 900xc2x0 F., and is cooled to a temperature of about 500 to 550xc2x0 F. in a kettle reboiler type shell and tube heat exchanger 58. The kettle reboiler 58 has a U-tube bundle 60, schematically shown as tube, immersed in a pool of water 62 on the shell side, with the hot compressor air in the tube. The hot air flowing in the tubes causes boiling in the pool 62 and the saturated steam 64 produced is admitted to the intermediate pressure (IP) evaporator 48 steam drum in the heat recovery steam generator (HRSG) 32. IP economizer discharge water 66 is admitted to the shell of the reboiler 58 as makeup for the steam 64 generated. The cooled air 68 leaving the CCA heat exchanger 58 is used for cooling compressor and/or turbine parts, as schematically shown by conduits 70 and 72. A by-pass 74 on the air side is also provided to control the air temperature leaving the CCA system 54. Since the CCA system has to be designed for low system air side pressure drop, for power cycle thermal efficiency considerations, a two pass U-tube bundle is typically used.
There are several potential areas for improvement in regard to the CCA system described above.
The two pass tube side design described above results in one half of the tube-sheet being exposed to the incoming hot air, while the other half is exposed to cooled air leaving the heat exchanger on the air-side. This results in high thermal stresses on the tube sheet. While a single pass kettle reboiler would overcome this issue, a single pass design would require a shell expansion joint or a sliding tube sheet, which are a reliability concern.
Accurate water level control on the shell-side is required to prevent excessive carry over of water in the saturated steam and to prevent the uncovering of tubes in the water pool and exposing water to the hot air tubes only transiently. In that regard, drum level is typically calculated with measured differential pressure in the drum and calculated density of the drum water. However, there are steam bubbles in the boiling pool of water and an accurate calculation of the void fraction is required for an accurate density and level calculation. Calculation of void fraction is challenging in this type of application, especially in transient situations.
In addition, as noted above, the air side has to be designed for low pressure drop. Hence, the velocity of the air in the tubes is low resulting in low heat transfer coefficients on the air side. Heat exchanger size can be advantageously reduced by applying extended surfaces (such as fins) on the low heat transfer side. Extended surfaces can be economically applied on the outside tube surfaces.
The invention provides a CCA system for application with modern gas turbine cycles that overcomes the reliability, control and cost concerns identified above.
More specifically, in an embodiment of the invention, the CCA system includes a shell and tube heat exchanger in which water flow is provided inside the tubes and air flow is provided for on the shell side of the heat exchanger. In such a system, the water exiting the heat exchanger is partially evaporated. Accordingly, as a further feature of the invention, the resulting two phase water/steam flow is admitted to a separator/flash drum where the steam and water are separated. The saturated steam leaving the separator is flowed to the HRSG whereas the separator water is admitted to the heat exchanger, generally after being pumped to a higher pressure by recirculating water pumps.
Because air flow is provided outside the tubes, finned tubes may be provided to reduce the total heat exchanger size.
Thus, the invention is embodied in a combined cycle power plant comprising a combustion turbine system having a compressor for producing compressed air, a combustor for combusting a fuel in the compressed air to produce combustion air, and a gas turbine for expanding the combustion air to produce mechanical energy and exhaust gas; a steam generator having an inlet for receiving the exhaust air and a plurality of sections located sequentially in a flow path of the exhaust gas for removing heat from the exhaust gas to produce at least one steam flow; a steam turbine system for receiving the at least one steam flow; and a cooling air flow path for directing a compressed air fraction from the compressor to at least one of the compressor and the gas turbine for cooling a portion thereof; wherein the cooling air flow path comprises a heat exchange system for receiving the compressed air fraction and for removing heat therefrom to produce a heated fluid flow and a cooled compressed air flow, wherein the heat exchange system comprises a chamber having a compressed air inlet and a compressed air outlet and at least one tube for flowing water for heat exchange with hot compressed air disposed in the chamber.
The invention is also embodied in a method of operating a combined cycle power plant having a combustion turbine system, a heat recovery steam generator, and a steam turbine system, the method comprising the steps of providing a heat exchange system comprising a chamber having a compressed air inlet and a compressed air outlet and at least one tube for flowing water for heat exchange with hot compressed air disposed in the chamber; operating the combustion turbine system to burn a fuel to produce hot compressed air, mechanical energy, and a flow of exhaust gas; directing a portion of the hot compressed air through the heat exchange system to produce a flow of cooled compressed air and a heated fluid flow; and directing the flow of cooled compressed air to cool a portion of the combustion turbine system.